1. Field of the Invention
The present invention relates to a robot control device for controlling a robot that performs work by pressing a tool against a workpiece.
2. Description of the Related Art
FDS (flow drilling screw) is known as a mechanical joining method for mechanically joining a plurality of different kinds of metal members. The joining method is a technique in which a screw rotating at high speed is drilled into the metal members laminated on each other to weld the metal members to each other. Such a mechanical joining method as FDS does not generate any compound layer between the metal members, and thus is advantageous in terms of joining a plurality of different kinds of metal members to each other.
In addition, it is sufficient for the mechanical joining method such as FDS to bring the screw close to the metal members laminated on each other in only one direction. Accordingly, the use of a robot allows the mechanical joining method to be easily automated. Such a mechanical joining method is thus used in automobile assembly processes and the like where multi-materialization is desired.
In an FDS process, the robot receives a counteractive force against a force applied when drilling a screw into the metal members laminated on each other. The counteractive force received by the robot twists a speed reducer of the robot and deforms an arm of the robot, as a result of which a position of a tool tip of the robot is deviated. The deviation of the position of the tool tip leads to deviation of a joining position of the metal members, and therefore joining quality may be lowered or joining work may fail.
In order to correct such a positional deviation of a tool tip, there are known methods for correcting twisting of a speed reducer of a robot. Japanese Patent No. 3654475 discloses a control technique for a motor provided with a speed reducer whose positioning can be highly accurately performed without being affected by a twist angle of the speed reducer. Specifically, a compensation value is calculated on the basis of a current value occurring due to twisting of the speed reducer to perform feedback control.
In addition, Japanese Laid-open Patent Publication No. H11-221707 discloses a technique for preventing deviation of a tip position of a tool by fastening the tool and a workpiece to each other.
The technique disclosed in Japanese Patent No. 3654475 presupposes that an arm of the robot is a rigid body and there is no friction at a tip of the tool. However, in fact, the arm is an elastic body and will be deformed by a counteraction from a workpiece. Furthermore, in fact, friction is generated in the tip position of the tool. Accordingly, in the technique of Japanese Patent No. 3654475, the deformation of the arm and a delay occurring due to the feedback control causes deviation of the tip position of the tool.
Once positional deviation of the tip of the tool occurs, it is necessary to apply a force larger than a maximum static friction force between a surface of a metal member and a surface of the tool contacted therewith in a direction opposite to a direction of the positional deviation. This makes the control of the robot extremely complicated.
In addition, the technique of Japanese Laid-open Patent Publication No. H11-221707 needs a component for fastening the tool and the workpiece to each other. This makes the structure complicated and can also increase cost.
The present invention has been accomplished in view of the circumstances. It is an object of the invention to provide a robot control device that allows positional deviation of a tip of a tool to be easily corrected without using a specific component.